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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="boost_process.faq"></a><a class="link" href="faq.html" title="Frequently Asked Questions">Frequently Asked Questions</a>
</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="faq.html#boost_process.faq.dead_lock">Why does this produce a
      deadlock?</a></span></dt>
<dt><span class="section"><a href="faq.html#boost_process.faq.closep">Why does the pipe not close?</a></span></dt>
<dt><span class="section"><a href="faq.html#boost_process.faq.wchar_t">When will the codecvt be used?</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_process.faq.dead_lock"></a><a class="link" href="faq.html#boost_process.faq.dead_lock" title="Why does this produce a deadlock?">Why does this produce a
      deadlock?</a>
</h3></div></div></div>
<p>
        Now let's revisit our c++filt example and we will put in an obvious mistake.
        This might however be not as obvious for more complex applications.
      </p>
<p>
</p>
<pre class="programlisting"><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">string</span><span class="special">&gt;</span> <span class="identifier">demangle</span><span class="special">(</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">string</span><span class="special">&gt;</span> <span class="identifier">in</span><span class="special">)</span>
<span class="special">{</span>

    <span class="identifier">ipstream</span> <span class="identifier">is</span><span class="special">;</span>
    <span class="identifier">opstream</span> <span class="identifier">os</span><span class="special">;</span>
    <span class="identifier">child</span> <span class="identifier">c</span><span class="special">(</span><span class="string">"c++filt"</span><span class="special">,</span> <span class="identifier">std_out</span> <span class="special">&gt;</span> <span class="identifier">is</span><span class="special">,</span> <span class="identifier">std_in</span> <span class="special">&lt;</span> <span class="identifier">os</span><span class="special">);</span>

    <span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">string</span><span class="special">&gt;</span> <span class="identifier">data</span><span class="special">;</span>
    <span class="keyword">for</span> <span class="special">(</span><span class="keyword">auto</span> <span class="special">&amp;</span> <span class="identifier">elem</span> <span class="special">:</span> <span class="identifier">data</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="identifier">string</span> <span class="identifier">line</span><span class="special">;</span>
        <span class="identifier">getline</span><span class="special">(</span><span class="identifier">is</span><span class="special">,</span> <span class="identifier">line</span><span class="special">);</span>
        <span class="identifier">os</span> <span class="special">&lt;&lt;</span> <span class="identifier">elem</span><span class="special">;</span>
    <span class="special">}</span>
<span class="special">}</span>
</pre>
<p>
        We switched the read and write operation up, so that's causing a dead-lock.
        This locks immediately. This is because <code class="computeroutput"><span class="identifier">c</span><span class="special">++</span><span class="identifier">filt</span></code>
        expects input, before outputting any data. The launching process on the other
        hand waits for its output.
      </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_process.faq.closep"></a><a class="link" href="faq.html#boost_process.faq.closep" title="Why does the pipe not close?">Why does the pipe not close?</a>
</h3></div></div></div>
<p>
        Now for another example, which might look correct, let's consider you want
        to use <code class="computeroutput"><span class="identifier">ls</span></code> to read the current
        directory.
      </p>
<p>
</p>
<pre class="programlisting"><span class="identifier">ipstream</span> <span class="identifier">is</span><span class="special">;</span>
<span class="identifier">child</span> <span class="identifier">c</span><span class="special">(</span><span class="string">"ls"</span><span class="special">,</span> <span class="identifier">std_out</span> <span class="special">&gt;</span> <span class="identifier">is</span><span class="special">);</span>

<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">file</span><span class="special">;</span>
<span class="keyword">while</span> <span class="special">(</span><span class="identifier">is</span> <span class="special">&gt;&gt;</span> <span class="identifier">file</span><span class="special">)</span>
    <span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"File: "</span> <span class="special">&lt;&lt;</span> <span class="identifier">file</span> <span class="special">&lt;&lt;</span> <span class="identifier">endl</span><span class="special">;</span>
</pre>
<p>
      </p>
<p>
        This will also deadlock, because the pipe does not close when the subprocess
        exits. So the <code class="computeroutput"><span class="identifier">ipstream</span></code> will
        still look for data even though the process has ended.
      </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
          It is not possible to use automatic pipe-closing in this library, because
          a pipe might be a file-handle (as for async pipes on windows).
        </p></td></tr>
</table></div>
<p>
        But, since pipes are buffered, you might get incomplete data if you do this:
      </p>
<p>
</p>
<pre class="programlisting"><span class="identifier">ipstream</span> <span class="identifier">is</span><span class="special">;</span>
<span class="identifier">child</span> <span class="identifier">c</span><span class="special">(</span><span class="string">"ls"</span><span class="special">,</span> <span class="identifier">std_out</span> <span class="special">&gt;</span> <span class="identifier">is</span><span class="special">);</span>

<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">file</span><span class="special">;</span>
<span class="keyword">while</span> <span class="special">(</span><span class="identifier">c</span><span class="special">.</span><span class="identifier">running</span><span class="special">())</span>
<span class="special">{</span>
    <span class="identifier">is</span> <span class="special">&gt;&gt;</span> <span class="identifier">file</span><span class="special">;</span>
    <span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"File: "</span> <span class="special">&lt;&lt;</span> <span class="identifier">file</span> <span class="special">&lt;&lt;</span> <span class="identifier">endl</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
      </p>
<p>
        It is therefore highly recommended that you use the asynchronous API if you
        are not absolutely sure how the output will look.
      </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_process.faq.wchar_t"></a><a class="link" href="faq.html#boost_process.faq.wchar_t" title="When will the codecvt be used?">When will the codecvt be used?</a>
</h3></div></div></div>
<p>
        Since windows does not use UTF-8 it is sometimes unavoidable to use the
        <code class="computeroutput"><span class="keyword">wchar_t</span></code> version of the WinApi.
        To keep this library consistent it provides <code class="computeroutput"><span class="keyword">wchar_t</span></code>
        support on posix also.
      </p>
<p>
        Since the posix api is purely <code class="computeroutput"><span class="keyword">char</span></code>
        every <code class="computeroutput"><span class="keyword">wchar_t</span></code> based type will
        be converted into <code class="computeroutput"><span class="keyword">char</span></code>.
      </p>
<p>
        Windows on the other hand is more selective; the default is to use <code class="computeroutput"><span class="keyword">char</span></code>, but if any parameter requires <code class="computeroutput"><span class="keyword">wchar_t</span></code>, everything will be converted to
        <code class="computeroutput"><span class="keyword">wchar_t</span></code>. This also includes
        <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">filesystem</span><span class="special">::</span><span class="identifier">path</span></code>. Additionally, if the system does
        not provide the <code class="computeroutput"><span class="keyword">char</span></code> api (as
        is the case with Windows CE) everything will also be converted.
      </p>
</div>
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      Felipe Tanus, Jeff Flinn, Boris Schaeling<br>Copyright © 2016 Klemens D. Morgenstern<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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